Biology Reference
In-Depth Information
Population variability in constructed landmarks will reflect population
variability in the traditional landmarks and in the surface used in
their construction.
2.2 Landmark homology and correspondence
In many biological studies that use landmark data, researchers refer
to landmarks as homologous . A homologue, as defined by Richard
Owen (1848), is “same organ in different animals under every variety
of form and function.” Though the definition seems simple enough, the
concept of homology continues to stimulate debate (Hall, 1994; 1999).
Some of this discussion stems from common usage of the term homol-
ogy as the “correspondence of parts” with no specification about
whether the parts correspond with respect to structure, phylogeny,
development, or on the basis of another relationship. Biologists (par-
ticularly paleontologists, anthropologists, and natural historians)
define homology as the establishment and conservation of individual-
ized structural units in organismal evolution (Müller and Wagner,
1996), or more simply, similarity in structure due to descent from a
common ancestor. When used in mathematics and engineering, homol-
ogy does not imply phylogenetic relationships. But even within a
discipline, the definition of homology is applied inconsistently, and a
practical definition remains elusive (Wagner, 1989; Roth, 1988).
Although we cannot cover the topic adequately here, the reader
needs to be aware of the nuances associated with the concept of homol-
ogy. For example, characters can appear to be homologous (similar due
to common descent) but actually represent homoplasy. Homoplasy
exists when a character is present in two species but is not possessed
by all intervening ancestors or by the most recent common ancestor of
the two or more species under consideration. A character can evolve
more than once in unrelated species due to similar selection pressures
acting on those species, as in the case of convergent evolution (e.g.,
wings evolved separately among the mammalia and among the birds;
leglessness evolved separately in snakes and in certain lizards).
This situation forces careful use of the term homology when select-
ing features for morphometric comparison. In this monograph, our
examples investigate differences in form within single species or with-
in very closely related species. However, care must be taken in the use
of the term homologous when landmarks designate features on organ-
isms of diverse phylogenetic history. We believe that the term
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